This invention relates to the treatment of fresh water sources to control and prevent infestation by zebra mussel. In one aspect, the invention relates to the treatment of fresh water intake pipes with concentrated solutions of aqueous chlorine dioxide.
The zebra mussel (biological name: Dreissena polymorpha) is a species of fresh water clam native to the Black and Caspian Seas and was accidentally introduced into the Great Lakes in 1986. These mollusks have migrated rapidly and the infestation has caused serious problems for water users and suppliers throughout the Great Lakes and adjacent water bodies. These problems are expected to only worsen in the future.
Water treating facilities, including municipal suppliers and industrial users, generally employ a water intake assembly known as a suction crib which is located offshore at distance ranging from 50 feet to 18,000 feet or more. The crib is a generally cylindrical structure with walls defined on the bottom by the lake bed, peripherally by a screen and rock pile formation formed around the screen, and on the top by a trash screen. The suction crib contains at its center a large diameter intake pipe for receiving fresh water and conducting the water to pumping and treating facilities onshore. It is common for eve large water treating facilities to draw all of its water from a single suction crib and intake pipe.
The infestation of the zebra mussel presents a serious problem to water treating facilities because they tend to adhere to submerged surfaces including metal, concrete, plastic, and even teflon. If left unchecked the mollusks will infest the intake crib, plug intake screens, restrict flow in the intake pipe, and affect water odor and taste.
The seriousness of the problem is described in the "Second International Zebra Mussel Research Conference" held in Rochester, N.Y. on Nov. 19-22, 1991, which was attended by more than three hundred registrants including research biologists, university students, consultants, end-users, and product and service representatives. As discussed at the conference, and highlighted in a paper entitled "Responding to the Zebra Mussel Threat--A Case History" by T. C. McTighe et al, methods for controlling zebra mussel infestation may be categorized as ecological, biological, mechanical, and chemical.
For various reasons including economy and long-term effectiveness (see the aforementioned paper for details), the chemical control method appears to offer the most feasible approach for controlling zebra mussel infestation of offshore water intake facilities. The chemical treatment involves introducing a biocide comprising an aqueous solution of oxidizing chemicals into the intake crib to destroy the mussels therein. These chemicals include free chlorine, potassium permanganate, chloramines, ozone, and hydrogen peroxide. Each of these chemicals have certain disadvantages which may limit their applicability in the treatment of potable or industrial water. For example, chlorine has the potential of forming carcinogenics such as trihalomethanes. Potassium permanganate is known to be less effective than the other oxidizers, while the use of ozone is not presently cost effective. Definitive research on the biological efficacy of chloramines and hydrogen peroxide are not well documented at the present time.
A presentation at the Second International Zebra Mussel Research Conference, referenced above, dealt with the effects of chlorine dioxide on Zebra Mussels. The only published part of the presentation is the Abstract which states
"chlorine dioxide treatment resulted in mortalities of about 10-20% at the 0.2-0.3 ppm level with 100% mortality reached in 24 hours at concentrations greater than 0.5 ppm . . . The high effectiveness of chlorine dioxide at inducing zebra mussel mortality, coupled with the fact that the addition of chlorine dioxide forms less THM's than chlorine and chloramine, indicates that it may be preferred as an oxidizing agent for zebra mussel control at water treatment plant intakes even if slightly higher concentrations are needed."
There, however, was no discussion or suggestion of the manner or method in which to inject the chlorine dioxide.
T. C. McTighe's paper referenced above discusses the problems associated with zebra mussel infestation and describes in detail a proposed design to control infestation at one specific location, the Shoremont Water Treatment Plant located on the southern shore of Lake Ontario in Rochester, N.Y. The plant is owned and operated by the Monroe County Water Authority (MCWA).
The Shoremont Water Treatment Plant is fed by an intake pipe extending from shore 8,200 feet into Lake Ontario and terminating in an open intake fixed within a suction crib on the lake bottom at a depth of 40 to 50 feet. Fresh water is drawn into the intake pipe by an onshore pumping station. To control zebra mussel infestation in the suction crib and intake pipe, a design was established for injecting a biocide directly into the crib. The plan that was adopted was to extend a pipeline of small diameter through the intake pipe from the onshore pump facility to the suction crib. The line would then serve to conduct the treating chemical to the crib and provide a continuous injection of the biocide. The biocide selected in the initial design was a combination of chlorine (sodium hypochlorite) and permanganate oxidizers. However, the MCWA has chosen to use only sodium hypochlorite. The treatment involves the use of dose rates ranging from 0.08 milligrams per liter to 1.8 milligrams per liter delivered continuously to the crib at an average flow rate of 65 gallons per day.
Although this method appears effective in controlling and preventing infestation of the zebra mussel, it is extremely expensive to install (e.g. approximately two million dollars for the MCWA plant). In addition, this method is limited to intake pipes which permit the insertion of the biocide chemical line therein.
In summary, the infestation of the zebra mussel poses a serious problem for water treatment facilities having offshore suction intake and there is currently a need for a flexible and cost effective biocidal process for controlling the infestation.